The invention relates to a binary-to-ternary converter for combining two binary signals into one ternary signal.
Such a binary-to-ternary converter is known from the German Patent Application No. 34 02 257, published Aug. 1, 1985. The binary-to-ternary converter disclosed therein comprises an electronic circuit having two inputs and an output transformer. In the Figure incorporated in this Patent Application the input for a first binary signal is referenced A.sub.1 and the input for a second binary signal is referenced A.sub.2. The secondary winding of the output transformer forms the output port of this binary-to-ternary converter. A positive pulse on input port A.sub.1 causes a positive output pulse on the output port. A positive pulse on input port A.sub.2 causes a negative output pulse on the output port. This known circuit thus combines two binary signals into one ternary signal.
Since transformers have disadvantages known to those skilled in the art, such as being relatively large and heavy, not being integrable and causing stray fields, transformers in electronic circuits are less desired.
When the output signal of a binary-to-ternary converter is transported via a long conductor, this binary-to-ternary converter should be resistant to overvoltages from outside. By serially connecting to the output port of the binary-to-ternary converter a protective resistance having a sufficiently high resistance value, in combination with a voltage-limiting means arranged at an appropriate location, sufficient protection is obtained. A practically suitable resistance value for this protective resistance is larger than 10 ohms.
Owing to the voltage loss through this protective resistance, the amplitude of the voltage of the output pulses is reduced. Even at the lowest resistance value of 10 ohms and a 75-ohm preset test load connected to the output and serially connected to the protective resistance, the amplitude of the voltage across the load is reduced by approximately 12%. As the protective resistance has a higher resistance value, which is generally desirable, so is the amplitude of the voltage across the load reduced further. With a like test load of 75 ohms and a protective resistance of 22 ohms, this amplitude is reduced by approximately 23%.
According to the CCITT Recommendation G703, a pulse of the ternary output signal at the output port shall have a voltage amplitude of 2.37 V.+-.10%. The minimum voltage amplitude is then 2.13 V, the maximum voltage amplitude 2.61 V.
In the binary-to-ternary converter known from the said German Patent Application No. 34 02 257 this minimum required amplitude is simple to realize, even if a protective resistance were serially connected to the output port. For, the voltage at the output port of this binary-to-ternary converter can be simply increased by increasing the number of windings on the secondary side of the output transformer. Serially connecting a protective resistor to the output port, causing a voltage loss, does not pose a problem with this binary-to-ternary converter.
When using a supply voltage of 5 V.+-.5% (as standardized for TTL logic, for example) a minimum supply voltage of 5 V-5%=4.75 V is to be taken into account. In this case, half the 4.75 V, thus 2.37 V remains for an output pulse of a ternary signal from a binary-to-ternary converter. If a protective resistance of 10 ohms (thus the lowest conceivable value) is serially connected to the output port of a binary-to-ternary converter, this resistor will cause, as calculated hereinbefore, a voltage loss of approximately 12% across a load having a 75-ohm resistance value. In that case there will remain 2.37 V-12%=2.09 V for the output pulse. The CCITT Recommendation G703, prescribing a minimum voltage amplitude of 2.13 V of the pulse at the output port, is then no longer satisfied. In practice 22 ohm protective resistors will rather be used. They cause a voltage loss of approximately 23%, as a result of which the voltage amplitude of a pulse at the output port is reduced even further. In this case the amplitude requirement of the CCITT Recommendation G703 is still further deviated from.
Under unfavourable operating conditions, the said known binary-to-ternary converter without a transformer is unable to furnish output pulses having the minimum prescribed 2.13 V voltage amplitude when using a standard supply voltage of 5 V.+-.5% and a protective resistance having a minimum resistance value of 10 ohms. In addition, the amplitude of these output pulses is even further reduced by the unavoidable voltage loss through the pulse shaping transistors at the output ports.